Heating a grow box, hot water bottle? help!

[havoksgrow]

Im currently growing in a 2foot tall box with 3 26watt cfls in a basement.
My room is around 20 C° during the day but its canada and winter so my rooms gets cold at night.
Ive pretty much spent all my money so im trying to find a free way to heat up my box at night.
I tried putting a hot water bottle in next to my plants and it seemed to warm it up. However my box now stinks like rubber... could this be bad for my plant?

 

[JJ Bones]

Do you have your lights turn on at night? Also what is the exact dimensions of the box.

 

[CronicHempHog]

lights on at night instead of during the day?

 

[KingJohnC]

I run my vegetative growth with a lighting schedule of 18/6 18 hours of light and 6 hours of darkness from 12:00 midnight to 6 pm, the light is on the longest at the coolest time of the day and off during the warmest part of the day. I run my flowering with a lighting schedule of 121/12 12 hours of light and 12 hours of darkness from 12:00 midnight to 12:00 noon the light is the longest during the coolest time of the day and off during the warmest part of the day. There are ceramic reptile heat lamps available in different wattage that may also help keep your temperature elevated in the cooler months.

https://www.google.com All Living Things Ceramic Heat Emitter

https://www.google.com All Living Things Ceramic Heat Emitter